Intermediate segment of an articulated arm comprising a screw and nut transmission

ABSTRACT

An intermediate segment ( 1 ) of a control or other arm, fitted with a transmission ( 7 ) for supporting a following segment ( 3 ) is made up of a motor ( 13 ) which is displaced towards the other end of the segment, preferably beyond the joint to a previous segment ( 5,2 ) in order to reduce the toppling moment on the segment. The transmission includes a movement conversion device using a screw ( 11 ) and nut ( 12 ).

The subject of this invention is an intermediate segment which includesa nut and screw transmission, and also an arm in which such anintermediate segment is put to use.

Articulated arms used as control arms or as working arms are mostcommonly made up of several segments which are articulated together andwhich are supported by powered transmissions involving so-called forcefeedback motors which retain the joints and segments by preventing themfrom falling when they are no longer being operated. Of thetransmissions that are known, some use a nut and screw system to convertthe rotation of a motor shaft into the translation of a longilinealdevice, such as a cable, which drives the next segment of the arm. Suchtransmissions offer several advantages, including simplicity,adaptability to joints of different shapes and lengths, precision ofmovement due to the small amounts of play and lightness. French patent 2809 464 illustrates such a transmission.

The motor is often found close to the nut, or integrated with screw-nutcylinder. The often preponderant weight of the motor results in atoppling moment and significant inertia in relation to the axes of thesegment joints.

This invention proposes a modification of this usual type of design inwhich, through the addition of a transmission, the motor is broughtcloser to the joint axis of the preceding segment in order to reduce theinertia and to exert an appropriate low toppling moment, or even inorder to contribute towards the equilibrium of the rest of thetransmission or segment. It then becomes very easy to hold or move thesegment, even when using a low-power motor.

In summary therefore, one aspect of the invention is an intermediatesegment of the articulated arm, fitted to a preceding segment by arotary joint and to which is fitted a following segment, including atransmission for holding the following segment, which is made up of amotor, a nut which rotates with the motor, a screw which is engaged inthe nut and a connection device stretching between the screw and thefollowing segment, characterised by the fact that the motor is connectedto the nut by an extended motor shaft so that the motor is in a positionwhich is close to the rotary joint axis. The motor shaft is in actualfact extended so that the motor is closer to the joint axis than thescrew and the nut. The motor may therefore be placed in front of thejoint, or beyond it, on the other side of the arm.

The invention will now be described in association with the figures:

FIG. 1 represents one option for manufacture of the invention,

FIG. 2 represents a slightly different option for manufacture,

and FIGS. 3 and 4 show two modes of manufacture for a control arm.

The portion of the arm represented in FIG. 1 by way of generalexplanation includes an intermediate segment 1 held at both ends by apreceding segment 2 and a following segment 3. The preceding segment 2is closer to a base, 4, and is here fixed to it. It is connected to theintermediate segment 1 by an initial joint 5, and the following segment3 is connected to the intermediate segment 1 by another joint 6. Thefollowing segment 3, which is further from the base 4 than intermediatesegment 1 may or may not include the end of the arm. Joints 5 and 6 arerotary joints, as are commonly used in this technical area, but otherconnections between segments 1, 2 and 3 would be possible. Here is howjoint 6 is controlled or maintained in a stopped position. Atransmission 7 is housed in the intermediate segment 1. It is made up ofa cable in the form of a loop, running between two pulleys 9 and 10,(the second of which actuates the joint 6), a screw 11 to which thecable 8 is attached, a nut 12 into which the screw 11 is engaged, and amotor 13. Rotation of the motor 13 causes the screw 12 to turn and movesthe screw 11 and cable 8 forward, causing the pulley 10 to rotate. Cable8 passes through the hollow screw 11, to which it is attached at apoint. Cable 8 is also attached to pulley 10, and rotation of screw 11is prevented by, for example, a roller trolley 14 fitted to screw 11 andwhich moves along a fixed slide 15.

The majority of the transmission 7, including the nut 12 and screw 11,are close to the second joint 6, and therefore exert a significanttoppling moment on segment 1 around the first joint 5 in the area of thesupport provided by the base 4. To counteract this, however, it is nowplanned to locate motor 13 from the rest of the transmission 7 to beplaced not far from the first joint 5 opposite the other joint 6, and inparticular in front of it or even beyond it in relation to the screw 11and nut 12. Thus the motor, which is usually the heaviest part of thetransmission 7, no longer exerts a significant toppling moment; if it isbeyond the first joint 5 it may even exert a toppling moment whichbalances that of the rest of the transmission 7 or of the intermediatesegment 1.

A long transmission shaft 16 can then be appropriately added between themotor 13 and a mechanism 17, such as a gear drive, in order to transmitthe motor rotation to the nut 12. The sub-transmission made up ofelements 16 and 17 in itself adds weight to the intermediate segment 1,but only a moderate amount, and this will often be compensated for bythe reduced weight of the motor 13 which could be achieved as a resultof the reduction in the toppling moment that has to be balanced. It mustalso be remembered that the decrease in weight and reduction in topplingmoment for a segment are advantageous not only for the design of thesegment involved, but also for the segments that precede it, which willalso have a smaller toppling moments applied to them. This will oftenenable the weight of their motors to be reduced, even if the inventionis not applied to them. Finally, the reduction of these forces will alsoallow the weight of the structure of the segments themselves to bereduced.

The device shown in FIG. 2 shows analogous components, although thetransmission, still identified as 7, controls a second so-calledrolling-motion joint 18 which makes the following segment 3 pivot,instead of a pitch motion joint as in FIG. 1. Cable 8 is then run in aslightly more complex manner over a set of pulleys 19 so that its endpasses around the following segment 3. Pulleys 19-1 and 19-2 areconcentric with the axis of joint in the following segment. Pulleys 19-3and 19-4 provide a change of direction or constraint. The movement ofthe transmission 7 is here also accompanied by movement in cable 8 andcauses the following segments 3 to rotate in relation to it. All theprevious considerations for transmission 7 remain valid.

FIG. 3 shows a control arm equipped with the invention. It is made up ofa base 20, a first segment 21 and second segment 22, a third segment 23and a grip 24. The first segment is connected to the base 20 by means ofa first joint (rolling-motion) 25, the second segment 22 is connected tothe first segment 21 by a second joint (double pitch-motion) 26, thethird segment is connected to the second segment 22 by a third joint(pitch-motion) 27, and the grip 24 is connected to the third segment 23by a fourth joint (double pitch-and rolling-motion) 28. The second joint26 is double, with the structure of the second segment 22 being in theform of a parallelogram: one degree of freedom causes the parallelogramto tip over, straightening the second segment 22, whilst another degreeof freedom alters the height of the parallelogram, and tips the thirdsegment 23 around the second pitch-motion joint 27. By this means aforce feedback motor associated with the third joint 27 may be placed inthe first segment 21, that is, at a position where it exerts no moment.

This arm with six degrees of freedom also includes six force feedbacktransmissions to stop all possible movements. These may all include nutand screw mechanisms, some of which are equipped with the invention.They bear references 29 to 34 and are located, respectively in the base20 to control the rotation of the first segment 21, in the first segment21 to control the pitch motion of the second segment 22 and that of thesecond segment 23, in the second segment 22, to control the rollingmotion of the third segment 23, and in the third segment 23 to preventthe pitch motion and rolling motion of the grip 24.

There is no need to apply the invention to the first three transmissions29, 30 and 31, the first of which is located in the fixed base 20 andthe another two in the first segment 21 which is vertical and whichpivots, so that they do not exert any toppling moment. The invention is,on the other hand, used to equip the other three transmissions 32, 33,and 34: the motor 13 in the fourth transmission 32 is located at therear of the second segment 22, beyond the second joint 26 and the motors13 of the fifth and sixth transmissions 33 and 34 are in a similarfashion located behind the third segment 23 and the third joint 27, nearto the second segment 22. Transmission shafts 16 and mechanisms 17 forthe transmission of movement to the nuts 12 are also shown. Three motors13 have therefore been moved, making it possible to completely balancingat least the second segment 22, if necessary by adding a balancingweight 35 close to the motor 13 for the fourth transmission 32 (or evena spring): the moment which the motors in the second and thirdtransmissions 30 and 31 must balance is thus greatly reduced, whichallows their weight, and possibly that of the rest of the arm up to thebase 20, to be reduced. The decrease in the inertia of the arm as aresult of this weight reduction will be particularly beneficial for amanipulated arm. The cumulative character of the effects of theinvention (which are more noticeable for arms made up of chains ofsegments) must be emphasised.

Certain variants of the arm remain possible which retain the propertiesof previous assemblies. It is already foreseen that the firsttransmission 29 could also be placed in the first segment 21. Moresignificant changes can be described by referring to FIG. 4. Theseinvolve, in an arm that is otherwise little different from that in FIG.3, the second segment, here referred to as 22′, and with the thirdtransmission 31′ which is responsible for applying a rolling-pitchingmotion to the third segment 23.

This third transmission 31′ is now located in the second segment 22′,which no longer has a parallelogram structure, but which has a standard(tubular) linear structure. The third transmission 31′ is constructedlike the fourth 32 that is adjacent to it, with its motor 13 offsettowards the rear of the second segment 22′, beyond the second joint 26.Here also the arm is subjected to a low toppling moment. In this optionfor manufacture, as with the other, balancing of the third segment maybe supplemented using a counterweight or a spring 36.

1. Intermediate segment of an articulated arm, fitted to a precedingsegment by a rotary joint and to which the following segment is fitted,made up of a transmission for supporting the following segment whichincludes a motor, a nut which rotates with the motor, a screw which isengaged in the nut, and a connection device running between the screwand the following segment, characterised by the fact that the motor islinked to the nut by a motor shaft which is extended so that the motoris in a position which is close to the rotary joint axis.
 2. Articulatedarm made up of a first pivoting segment mounted on a base, a secondsegment mounted on the first segment, a third segment mounted on thesecond segment and a fourth segment, characterised by the fact that thesecond and third segments are intermediate segments as described inclaim 1.